Control Systems For Four-quadrant Cascaded Multilevel High-voltage High-power Inverters

Compared with a conventional cascaded multilevel inverter (CMI) using the diode rectifiers and limiting regeneration operation, a four-quadrant CMI using the voltage source PWM rectifiers enables a bidirectional power flow and finds wider application fields than the classical one. A CMI synthesizes a high-voltage output based on series connection of power cells, each consisting of a voltage source PWM rectifier at the line side, a dc capacitor, and a H-bridge inverter at the motor side. Each voltage source PWM rectifier at the line side needs to be individually controlled, while all the H-bridge inverters are subject to unified modulation. The four-quadrant CMIs are mainly applied in variable frequency speed control field of high-voltage high-power ac electric machines.This dissertation studies the four-quadrant CMI from three aspects: the control of the voltage source PWM rectifiers at the line side, the modulation of the H-bridge inverters at the motor side, and its application in the speed control systems of high-voltage high-power ac electric machines. All the control strategies and modulation methods presented here are verified to be valid by simulation analysis. The main research fields and results are as follows.(1) The nonlinear control strategy for the voltage source PWM rectifier at the line sideTwo nonlinear control strategies for the three phase voltage source PWM rectifier are put forward. For the first strategy, the sliding mode control was applied to the inner current control loop, and a sliding mode controller was designed according to the reaching law algorithm, which raised the robustness of the system rejecting the disturbance of the parameters. The advantages of the first strategy lie in the simple design and the easy implementation. The control of the outer voltage loop combines the feed-forward control of the load current and the feedback control of the output dc voltage. This combination brings the advantage into full play of the timely operation of the feed-forward control, while keeps the other advantage of the feedback control rejecting other disturbances and checking the feedback of the controlled variables. For the second strategy, a voltage controller and a current controller were designed by the active disturbance rejection control (ADRC) technique, and the corresponding control system manifests high robustness in rejecting disturbances from the load and some other parameters. (2) The PWM technique for the H-bridge inverters at the motor sideSpace vector control technique was respectively applied to modulating a CMI using five-level H-bridge inverters and an asymmetric CMI. This modulation method enables a CMI to generate almost sinusoidal voltage at low switching frequency and low common-mode voltage, thus effectively reducing switching losses, and raising conversion efficiency.(3) The application of the CMI in the speed control system of an asynchronous machineA vector control scheme was put forward for an asynchronous machine driven by a CMI. The scheme involves the design of a speed controller by an ADRC technique, the generation of the switching signals by the carrier phase-shifted PWM method, and the using of an adaptive observer for the rotor flux. The simulation results have shown that the system has good dynamic and static performance.A direct torque control scheme was presented for an asynchronous machine driven by a CMI. The reference voltage vector was obtained by the direct torque control based on the radial flux trajectory, the switching signals for the CMI were generated by the carrier phase-shifted PWM method, and the stator flux was observed by an adaptive observer. The simulation results have shown that at any speed within the limit, the pulsation of the torque is small.(4) The application of CMI in the speed control system of a permanent magnet synchronous machineA vector control scheme was put forward for a permanent magnet synchronous machine. The scheme uses the sliding mode control technique and the ADRC technique to realize the high performance control of a permanent magnet synchronous machine driven by two-level inverter or CMI. The simulation results have shown that the system has good dynamic and static performance.A direct torque control scheme was designed for the permanent magnet synchronous machine. In this scheme, an adaptive stator flux observer was presented that can quickly and exactly estimate both the stator flux and the stator resistance. A speed controller was designed based on the ADRC technique that estimates and compensates the load disturbance. The reference voltage vector was obtained by a direct torque control technique based on the control of the load angle. The switching signals for the CMI were generated by the earner phase-shifted PWM method. The simulation results have shown that at any speed within the limit, the pulsation of the torque is small.(5) The space vector PWM of a neutral-point-clamped three level inverterA space vector PWM method was presented for a neutral-point-clamped three level inverter with the balancing control of the neutral-point potential. The method only needs to check the direction of the neutral-point current and the capacitor voltage. The neutral-point potential is controlled by rearranging the time distribution of the redundant small voltage vectors. The method only involves some simple calculations. This space vector PWM method can be easily applied as long as the PWM rectifiers at the line side are neutral-point-clamped three level ones.